Optical deflector, optical scanning apparatus, and image forming apparatus

ABSTRACT

An optical deflector including a rotary member supported by a bearing shaft and rotatively driven by a motor for deflecting a plurality of laser beams separated from each other in a rotational axis direction of the rotary member is disclosed. The optical deflector includes a polygon mirror having four sides arranged about the rotational axis direction. Each of the four sides is a continuous plane having a plurality of effective reflection areas separated from each other in the rotational axis direction.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical deflector, an opticalscanning apparatus, and an image forming apparatus used for, forexample, forming color images.

2. Description of the Related Art

Japanese Laid-Open Patent Application No. 2-16521 discloses an exampleof a related art case of an optical deflector (deflecting apparatus)used in, for example, a color image forming apparatus. The deflectingapparatus includes: plural polygon mirrors for deflecting plural lightbeams, respectively; a connecting part formed equal to or smaller thanan inscribed circle of the plural polygon mirrors for connecting theplural polygon mirrors; and a drive motor that drives rotatively. Thus,the polygon mirrors form a united body having a two level configurationwith one polygon mirror on top of the other. However, since the polygonmirrors are fixed to a rotating shaft by a leaf spring member, thetemperature rise and centrifugal force generated by high speed rotationof the rotating shaft causes the fixed polygon mirrors to slightly shiftand change the balance of the rotated configuration. This results in theproblem of vibration. Particularly, a significant vibration may becreated even by a slight shift of the polygon mirrors since the mass ofthe polygon mirrors having the two-level configuration is heavy.

The optical deflector used in a color image forming apparatus includes atype configured to have plural laser beams incident thereto and deflectthe plural incident laser beams. Therefore, this type of opticaldeflector either has the reflection surface of the polygon mirror formedwith a large area (increasing thickness of mirror, thick mirrorconfiguration) or has two separate mirrors spaced apart from each otherin the axis direction (double mirror configuration). Meanwhile, apolygon scanner having the thick mirror configuration or the doublemirror configuration is to be rotated at a high speed of 25,000 rpm ormore with high precision for enabling a color image forming apparatus toachieve high speed printing and provide image quality of highdefinition. However, as the thick mirror configuration is rotated athigh speed, windage loss created by the mirror becomes greater. As aresult, the windage loss causes the electric power consumption of themotor to increase. Furthermore, with the double mirror configuration,the reference plane during the mounting of the mirrors is required to beprocessed (finished) with high precision. Furthermore, the optical facetangle between the two mirrors is also required to be set with highprecision. Such complicated process/assembly steps lead to a problem ofincreased manufacturing cost of the double mirror configuration.

Furthermore, as described above, in a case where the configurationhaving two levels of polygon mirrors forming a united body is rotated athigh speed, the temperature rise and centrifugal force generated by thehigh speed rotation cause the polygon mirrors to slightly shift, changethe balance of the rotated configuration, and result in the problem ofvibration. This is caused by the polygon mirrors being fixed to arotating shaft by a leaf spring member. Particularly, a significantvibration may be created even by a slight shift of the polygon mirrorssince the mass of the polygon mirrors having the two-level configurationis large. Furthermore, the slight shifting (balance change of therotated configuration) and the resulting increase of vibration occurringwhen rotating the configuration at a high speed in a high temperatureenvironment are caused by, for example, the different coefficients ofthermal expansion of the components of the rotated configuration(polygon mirror, a flange fixing a rotary magnet, rotating shaft) or(even in a case where the coefficients of thermal expansion of thecomponents match) tolerance and/or the method of the fixing components.

With respect to the above-described problem, Japanese Laid-Open PatentApplication No. 2003-177346 (filed by applicant) discloses thebelow-described optical deflector for providing a polygon scanner andits processing method for achieving high speed rotation with lowvibration at high temperature as well as reduction of power consumptionand facilitation of assembly.

In the optical deflector disclosed in Japanese Laid-Open PatentApplication No. 2003-177346, a rotary member 8 of a polygon scanner 1includes polygon mirror reflecting surfaces 8 a, 8 b separated in anaxial direction, a circumferential surface fixing a rotary magnet 11 ofan outer rotary motor, and a circumferential surface fixing a bearingshaft 10, in which the polygon mirror reflecting surfaces 8 a, 8 b andthe circumferential surfaces are formed of a single member. The rotarymember 8 has circumferential grooves 8 h, 8 i, and 8 k which are used asadhesive coating parts for balance correction and prevention of stressstrain against the reflecting surfaces 8 a, 8 b that occur duringshrinkage fitting of the bearing shaft 10 or in correspondence withchanges of temperature of the environment. Furthermore, the polygonmirror reflecting surfaces 8 a, 8 b have upper and lower surfaces of asubstantially center part shaped substantially as a concave or a convexpart with respect to both ends of a part contributing to deflection in amain scanning direction, to thereby prevent deviation of respectivecolors of a color image forming apparatus.

Although the optical deflector disclosed in Japanese Laid-Open PatentApplication No. 2003-177346 is able to reduce windage loss of themirrors that have their sizes increased along with the increase in highspeed rotation by forming a two level configuration by removing anintermediate part of the separated reflecting surfaces, it is difficultto increase the high speed rotation further. In addition, environmentalburdens such as power consumption and noise pollution are becominggreater as the high speed rotation is further increased.

Furthermore, although the optical deflector disclosed in JapaneseLaid-Open Patent Application No. 2003-177346 is provided with a polygonmirror having 5, 6, or more surfaces for increasing the number of scansper rotation and accelerating printing speed, the reflection surface isto have its width increased to some extent in order to obtain a desiredscanning width for the image forming area of the light beam. Thisresults to a problem where the size of the radius of the inscribedcircle of the polygon mirrors becomes large. This leads to increase thewindage loss of the mirrors.

Furthermore, due to the above-described increase in the radius of theinscribed circle of the polygon mirrors, deformation of the mirrorscaused by centrifugal force becomes greater as rotational speed isincreased. This results in deterioration of profile regularity.Furthermore, the starting time for the mirrors to reach a predeterminednumber of rotations (rpm) for scanning becomes longer the more therotational speed is increased. This increases the amount of powerconsumed before reaching an actual operating state.

SUMMARY OF THE INVENTION

The present invention may provide an optical deflector, an opticalscanning apparatus, and an optical deflector, that substantiallyobviates one or more of the problems caused by the limitations anddisadvantages of the related art.

Features and advantages of the present invention are set forth in thedescription which follows, and in part will become apparent from thedescription and the accompanying drawings, or may be learned by practiceof the invention according to the teachings provided in the description.Objects as well as other features and advantages of the presentinvention will be realized and attained by an optical deflector, anoptical scanning apparatus, and an image forming apparatus particularlypointed out in the specification in such full, clear, concise, and exactterms as to enable a person having ordinary skill in the art to practicethe invention.

To achieve these and other advantages and in accordance with the purposeof the invention, as embodied and broadly described herein, anembodiment of the present invention provides an optical deflectorincluding a rotary member supported by a bearing shaft and rotativelydriven by a motor for deflecting a plurality of laser beams separatedfrom each other in a rotational axis direction of the rotary member, theoptical deflector including: a polygon mirror having four sides arrangedabout the rotational axis direction; wherein each of the four sides is acontinuous plane having a plurality of effective reflection areasseparated from each other in the rotational axis direction.

In the optical deflector according to an embodiment of the presentinvention, the polygon mirror may be formed of a material having Young'smodulus in the range 60-220 GPa.

In the optical deflector according to an embodiment of the presentinvention, aluminum may be a main constituent of the material of thepolygon mirror.

In the optical deflector according to an embodiment of the presentinvention, the four sides of the polygon mirror may be reflectionsurfaces that are formed by surface processing a material having Young'smodulus in the range 200-400 GPa.

In the optical deflector according to an embodiment of the presentinvention, the polygon mirror may have a center hole, wherein the ratioof the diameter of the center hole with respect to a circumscribedcircle diameter is in the range 10-40%.

In the optical deflector according to an embodiment of the presentinvention, the four sides of the polygon mirror may be reflectionsurfaces that are formed by surface processing a material having Young'smodulus in the range 1-10 GPa.

In the optical deflector according to an embodiment of the presentinvention, the polygon mirror may be fixed to the rotary member by anengagement portion formed at an outer side of the polygon mirror.

In the optical deflector according to an embodiment of the presentinvention, the engagement portion may be an outer ridge line part of thepolygon mirror.

Furthermore, another embodiment of the present invention provides anoptical scanning apparatus including an optical system for scanning ascanning line on a target scanning surface by guiding a beam of asemiconductor laser to the target scanning surface and forming a beamspot on the target scanning surface, the optical scanning apparatusincluding: the optical deflector according to an embodiment of thepresent invention for deflecting the beam to the target scanningsurface.

Furthermore, another embodiment of the present invention provides anoptical scanning apparatus including an optical system for scanning aplurality of scanning lines on a target scanning surface by guiding aplurality of beams of a semiconductor laser to the target scanningsurface and forming a plurality of beam spots on the target scanningsurface, the optical scanning apparatus including: the optical deflectoraccording to an embodiment of the present invention for deflecting theplural beams to the target scanning surface.

Furthermore, another embodiment of the present invention provides animage forming apparatus for forming a visible image from a latent imageformed on a photoconductor by scanning a light beam on a photosensitivesurface of the photoconductor, the image forming apparatus including:the optical scanning apparatus according to an embodiment of the presentinvention.

Other objects and further features of the present invention will beapparent from the following detailed description when read inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram for describing a laser beam scanningperformed by using a six sided polygon mirror and a four sided polygonmirror;

FIG. 2 is a schematic diagram showing a six sided polygon mirror and afour sided polygon mirror having equal reflection surface width A;

FIG. 3 is a graph showing the flatness of a six sided polygon mirrorhaving a circumscribed circle diameter of approximately 42 mm (material:high purity aluminum);

FIG. 4 is a graph showing the flatness of a four sided polygon mirrorhaving a circumscribed circle diameter of approximately 20 mm (material:high purity aluminum);

FIG. 5 is a graph showing the flatness of a four sided polygon mirrorhaving a circumscribed circle diameter of approximately 20 mm (material:polycarbonate);

FIG. 6 is a graph showing deformation from centrifugal force for a foursided polygon mirror having a circumscribed circle diameter ofapproximately 20 mm (material: polycarbonate);

FIG. 7 is a graph showing characteristic values used in calculatingdeformation from centrifugal force;

FIG. 8 is a vertical cross-sectional view showing an optical deflectoraccording to a related art;

FIG. 9A is a vertical cross-sectional view showing an optical deflectoraccording to an embodiment of the present invention;

FIG. 9B is a perspective view showing an optical deflector according toan embodiment of the present invention;

FIG. 10 is a perspective view showing a configuration of an opticalscanning apparatus according to an embodiment of the present invention;

FIG. 11 is a perspective view showing a configuration of a multi-beamoptical scanning apparatus according to an embodiment of the presentinvention; and

FIG. 12 is a cross-sectional view showing a configuration of an imageforming apparatus (tandem type full color laser printer) according to anembodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following, embodiments of the present invention will be describedwith reference to the accompanying drawings.

First, the characteristics of an optical deflector, an optical scanningapparatus, and an image forming apparatus according to an embodiment ofthe present invention are described below.

The optical deflector according to an embodiment of the presentinvention includes a rotary member supported by a bearing shaft androtatively driven by a motor for deflecting a plurality of laser beamsseparated from each other in a rotational axis direction of the rotarymember. The optical deflector has a polygon mirror having four sidesarranged about the rotational axis direction, wherein each of the foursides is a continuous plane having a plurality of effective reflectionareas separated from each other in the rotational axis direction.Accordingly, with the optical deflector according to an embodiment ofthe present invention, environmental burdens from the manufacturingstage to the operating stage (e.g. consumption of material and energy)can be reduced. Furthermore, in the manufacturing stage, the material ofthe polygon mirror as well as the energy consumption for processing thepolygon mirror can be reduced. Moreover, in the operating stage, thepower consumption during the starting period of the rotary member can bereduced by shortening the starting time of the rotary member.Furthermore, in the operating stage, windage loss of the mirror, powerconsumption during a steady rotating state, and noise can be reduced.

In the optical deflector according to an embodiment of the presentinvention, the polygon mirror is formed of a material having Young'smodulus in the range 60-220 GPa. Accordingly, with the optical deflectoraccording to an embodiment of the present invention, deformation of thereflection surfaces of the polygon mirror caused by the centrifugalforce can be reduced, and the polygon mirror can be easily processed.

In the optical deflector according to an embodiment of the presentinvention, aluminum is a main constituent of the material of the polygonmirror. Accordingly, with the optical deflector according to anembodiment of the present invention, the mass of the polygon mirror andthe moment of inertia of the rotary member can be reduced. Thereby, thestarting time of the rotary member can be shortened.

In the optical deflector according to an embodiment of the presentinvention, the four sides of the polygon mirror are reflection surfacesthat are formed by surface processing a material having Young's modulusin the range 200-400 GPa. Accordingly, with the optical deflectoraccording to an embodiment of the present invention, deformation of thereflection surfaces of the polygon mirror caused by the centrifugalforce can be reduced, and the polygon mirror can be resistant toscratches. Thereby, the optical deflector according to an embodiment ofthe present invention can be easily recycled.

In the optical deflector according to an embodiment of the presentinvention, the polygon mirror has a center hole in which the ratio ofthe diameter of the center hole with respect to a circumscribed circlediameter is in the range 10-40%. Accordingly, with the optical deflectoraccording to an embodiment of the present invention, deformation of thereflection surfaces of the polygon mirror caused by the centrifugalforce can be reduced.

In the optical deflector according to an embodiment of the presentinvention, the four sides of the polygon mirror are reflection surfacesthat are formed by surface processing a material having Young's modulusin the range 1-10 GPa. Accordingly, with the optical deflector accordingto an embodiment of the present invention, the base material of thepolygon mirror can be easily processed (fabricated) by molding resin,and the mass of the polygon mirror can be reduced. Thereby, the startingtime of the rotary member can be shortened.

In the optical deflector according to an embodiment of the presentinvention, the polygon mirror is fixed to the rotary member by anengagement portion formed at an outer side of the polygon mirror.Accordingly, with the optical deflector according to an embodiment ofthe present invention, the small-sized polygon mirror can easily bepositioned and fixed to said position.

In the optical deflector according to an embodiment of the presentinvention, the engagement portion is an outer ridge line part of thepolygon mirror. Accordingly, with the optical deflector according to anembodiment of the present invention, the polygon mirror requires noengagement portion dedicated for positioning the polygon mirror andfixing the polygon mirror to said position.

Furthermore, an optical scanning apparatus according to an embodiment ofthe present invention includes an optical system for scanning a scanningline on a target scanning surface by guiding a beam of a semiconductorlaser to the target scanning surface and forming a beam spot on thetarget scanning surface. The optical scanning apparatus has theabove-described optical deflector for deflecting the beam to the targetscanning surface. Accordingly, with the optical scanning apparatusaccording to an embodiment of the present invention, the reflectionsurfaces of the polygon mirror of the optical deflector can maintain ahighly precise configuration, the scanning beam can maintain constantshape, noise can be reduced, and environmental burdens from themanufacturing stage to the operating stage (e.g. consumption of materialand energy) can be reduced.

Furthermore, an optical scanning apparatus according to an embodiment ofthe present invention includes an optical system for scanning aplurality of scanning lines on a target scanning surface by guiding aplurality of beams of a semiconductor laser to the target scanningsurface and forming a plurality of beam spots on the target scanningsurface. The optical scanning apparatus includes the above-describedoptical deflector for deflecting the plural beams to the target scanningsurface. Accordingly, with the optical scanning apparatus (multi-beamoptical scanning apparatus) according to an embodiment of the presentinvention, the reflection surfaces of the polygon mirror of the opticaldeflector can maintain a highly precise configuration, the scanningbeams can maintain constant shape, noise can be reduced, andenvironmental burdens from the manufacturing stage to the operatingstage (e.g. consumption of material and energy) can be reduced.

Furthermore, an image forming apparatus according to an embodiment ofthe present invention for forming a visible image from a latent imageformed on a photoconductor by scanning a light beam on a photosensitivesurface of the photoconductor includes the above-described opticalscanning apparatus. Accordingly, with the image forming apparatusaccording to an embodiment of the present invention, the scanning beamsof the optical scanning apparatus can maintain a constant shape, highquality images can be obtained, noise can be reduced, and environmentalburdens from the manufacturing stage to the operating stage (e.g.consumption of material and energy) can be reduced.

Next, an optical deflector according to an embodiment of the presentinvention is described in further detail below.

[Comparison of Mirror Windage Loss]

In a related art case, a two-level polygon mirror having six sides and acircumscribed circle diameter (i.e. diameter of the circumscribed circleof the polygon mirror) of approximately 42 mm is used in a color imageforming apparatus (e.g. an A3 size copier, a printer) for scanning alength (distance) of approximately 300 mm on a photoconductor. Insteadof using the six sided polygon mirror, the optical deflector 1000according to an embodiment of the present invention uses a polygonmirror 1001 having four sides and a circumscribed circle diameter ofapproximately 20 mm. Furthermore, the polygon mirror 1001 of the opticaldeflector 1000 according to an embodiment of the present invention hasplural effective reflection areas formed on a single continuous planethereof in a manner separated from each other in a rotational axisdirection. The use of the optical deflector 1000 according to anembodiment of the present invention reduces environmental burdens (e.g.consumption of energy and material) from the manufacturing process tothe operating process.

FIG. 1 is a schematic diagram for describing the use of a four sidedpolygon mirror instead of a six sided polygon mirror in a case of havinga laser beam reflected from a polygon mirror rotating in acounter-clockwise direction and having the reflected laser beam scan aphotoconductor from a right end to a left end.

The reflection surface of the six sided polygon mirror is positionedsubstantially at the same position as that of the four sided polygonmirror and scans the same scan width as the four sided polygon mirror.In order to match the position of the reflection surface of the sixsided polygon mirror with the position of the reflection surface of thefour sided polygon mirror, the rotation center of the six sided polygonmirror is situated at a position different from the rotation center ofthe four sided polygon mirror. The size required for scanning the samewidth is significantly different between the six sided polygon mirrorand the four sided polygon mirror. That is, the four sided polygonmirror can be formed in an extremely small size compared to the sixsided polygon mirror even where the scanning width is the same.

As shown in FIG. 1, in a case of scanning a laser beam from a right endto a left end of a photoconductor, a laser beam reflection positionmoves from a left end to a right end of the polygon mirror. The movementof the laser beam reflection position on the polygon mirror is greaterfor the six sided polygon mirror than that of the four sided polygonmirror. That is, the six sided polygon mirror has a wider reflectionsurface than that of the four sided polygon mirror. Since the distancebetween the rotation center and the reflection surface is smaller forthe four sided polygon mirror than the six sided polygon mirror, themovement of the six sided polygon mirror becomes greater than that ofthe four sided polygon mirror with respect to the same amount of changein the rotation angle.

FIG. 2 is a schematic view showing a six sided polygon mirror and a foursided polygon mirror superposed on each other where the six sidedpolygon mirror and the four sided polygon mirror have a reflectionsurface of equal width (hereinafter referred to as “reflection surfacewidth A”). FIG. 2 shows that the diameter of the circumscribed circle ofthe four sided polygon mirror (i.e. circumscribed circle diameter) issmaller than that of the six sided polygon mirror. Since the distancebetween the rotation center and the reflection surface can be shortenedfor the four sided polygon mirror, it is possible to further reduce thereflection surface of the four sided polygon mirror.

The six sided polygon mirror can be replaced with the four sided polygonmirror by using an optical layout design where, for example, the sixsided polygon mirror having a circumscribed circle diameter ofapproximately 42 mm is replaced with a four sided polygon mirror havinga circumscribed circle diameter of approximately 20 mm. In a case ofreplacing the six sided polygon mirror with the four sided polygonmirror, the rotation speed along the circumference of circumscribedcircle can be reduced to 0.48 (20/42) times since the rotation speedreduction of the four sided polygon mirror is in proportion to thecircumscribed circle diameter of the six sided polygon mirror and thefour sided polygon mirror. However, in order to obtain the same scanningspeed as the six sided polygon mirror, the four sided polygon mirror isto be rotated at a rotation speed of 1.5 (6/4) times. Therefore, in acase of replacing the six sided polygon mirror with the four sidedpolygon mirror, the rotation speed along the circumference of thecircumscribed circle of the four sided polygon mirror is reduced to 0.72times (0.48×1.5).

Since air resistance is typically in proportion to the second power ofrotation speed, the air resistance along the circumscribed circle of thefour sided polygon mirror is reduced 0.52 times with respect to the sixsided polygon mirror. Since the angle of the surface facing the wind isdifferent between the six sided polygon mirror and the four sidedpolygon mirror, the amount of air resistance that is reduced may becomeslightly less. Nevertheless, the four sided polygon mirror isadvantageous compared to the six sided polygon mirror in a case ofobtaining the same scanning speed. This advantage in the reduction ofair resistance is most effective for an optical deflector that deflectsplural laser beams separated from each other in a rotational axisdirection.

As shown in the above-described Japanese Laid-Open Application No.2003-177346, the optical deflector of the related art has a two-levelsix sided polygon mirror for deflecting plural laser beams separatedfrom each other in a rotational axis direction. With this configuration,however, air resistance during the rotation of the six sided polygonmirror becomes extremely large. Therefore, an intermediate part betweenits plural reflection surfaces separated from each other in therotational axis direction is cut out (machined) so as to reduce airresistance. Hence, the six sided polygon mirror of the related artrequires a step of cutting off the intermediate part and also requires alarge amount of material to be consumed. Furthermore, the six sidedpolygon mirror requires performing a step of finishing (processing) onsix sides so as to form six mirror surfaces. As a result, environmentalburdens are large in the manufacturing stage (e.g. consumption of energyand material). In addition, energy consumption and noise created by themirrors against the air also are large when the six sided polygon mirroris in an operating state (operation stage). Furthermore, the more thesix sided polygon mirror is rotated at higher speed, the longer becomesthe starting time for the six sided polygon mirror to reach a number ofrotations (rpm) for scanning (scanning rotations). Therefore, electricpower consumed when starting the optical deflector having the six sidedpolygon mirror becomes large.

The optical deflector 1000 according to an embodiment of the presentinvention is a four sided polygon mirror 1001 in which each side of thefour sided polygon mirror is a continuous plane having plural effectivereflecting areas 1001 a, 1001 b separated in a rotational axis direction(See FIG. 9A, 9B). By forming the optical deflector 1000 with such aconfiguration, the following advantages can be obtained.

As described above, with the four sided configuration of the polygonmirror 1001, the amount of air resistance during rotation of the polygonmirror 1001 can be reduced considerably. Therefore, unlike the six sidedpolygon mirror, there is no need to cut out an intermediate part betweenits plural reflection surfaces separated in the rotational axisdirection. Furthermore, since the polygon mirror 1001 according to anembodiment of the present invention can be formed in an extremely smallsize, the amount of material used in forming the polygon mirror 1001 canbe reduced. Furthermore, the step of processing the mirror surfaces isrequired to be performed only on four sides rather than on six sides.Furthermore, the area for processing each mirror surface on each side ofthe four sided polygon mirror 1001 is smaller than the area forprocessing each mirror surface on each side of the six sided polygonmirror. As a result, the four sided configuration of the polygon mirror1001 can reduce environmental burdens (e.g. consumption of energy andmaterial) during the stage of manufacturing the optical deflector 1000.Furthermore, the four sided configuration of the polygon mirror 1001 canalso reduce consumption of electric power and noise created by mirrorsmoving against the air during the stage of operating the opticaldeflector 1000. Furthermore, since the polygon mirror 1001 is formed ina small size and the moment of inertia of the rotary member 1002including the polygon mirror 1001 is small, the starting time for thepolygon mirror 1001 to reach a predetermined number of rotations(rotational speed) for performing the scanning can be shortened.Thereby, the electric power consumed when starting the optical deflector1000 can be reduced.

[Comparison of Deformation of Polygon Mirror Due to Centrifugal Force]

FIG. 3 is a graph showing the results obtained by calculatingdeformation due to centrifugal force (centrifugal force deformation)with respect to an optical deflector having a six sided polygon mirroraccording to a comparative example. The six sided polygon mirror of thecomparative example has a circumscribed circle diameter of approximately42 mm and is formed of high purity aluminum. When the six sided polygonmirror is rotated at high speed, a center part of its mirror surfaceprotrudes further outside than both ends, such that the mirror surfacedeforms into a convex shape. Thus, the flatness of the mirror isdeteriorated. The amount of deformation is influenced by the diameter ofthe center hole of the polygon mirror. That is, the mirror deformationfrom high speed rotation becomes greater the larger the center hole is,and becomes less the smaller the center hole is. Normally, in a case ofa six sided polygon mirror, a flatness of 0.32 μm or less is required(in a case of a high precision six sided polygon mirror, a flatness of0.16 μm or less is required).

FIG. 4 is a graph showing the results obtained by calculatingdeformation due to centrifugal force (centrifugal force deformation)with respect to an optical deflector 1000 having a four sided polygonmirror 1001 according to an embodiment of the present invention. Thefour sided polygon mirror 1001 according to an embodiment of the presentinvention has a circumscribed circle diameter of approximately 20 mm andis formed of high purity aluminum. When the four sided polygon mirror1001 is rotated at high speed, a center part of its mirror surfaceprotrudes further outside than both ends, such that the mirror surfacedeforms into a convex shape. Thus, the flatness of the mirror isdeteriorated. Although the amount of deformation is influenced by thediameter of the center hole of the polygon mirror 1001, the influence ofthe diameter of the center hole is less compared to that of the sixsided polygon mirror shown in FIG. 3. Normally, in a case of a foursided polygon mirror, a flatness of 0.15 μm or less is required (in acase of a high precision four sided polygon mirror, a flatness of 0.08μm or less is required).

In comparing the centrifugal force deformation of the polygon mirrors byreferring to FIGS.3 and 4, the absolute value of the amount ofdeformation of the four sided polygon mirror 1001 is smaller than thatof the six sided polygon mirror. That is, the four sided polygon mirror1001 is more effective against centrifugal force deformation.Furthermore, since the four sided polygon mirror 1001 is less influencedby the diameter of its center hole, the degree of freedom in designingthe diameter of the center hole of the four sided polygon mirror 1001 ishigher than that of the six sided polygon mirror.

FIG. 5 is a graph showing the results obtained by calculatingdeformation due to centrifugal force (centrifugal force deformation)with respect to an optical deflector 1000 having a four sided polygonmirror 1001 according to another embodiment of the present invention.The four sided polygon mirror 1001 according to this embodiment of thepresent invention has a circumscribed circle diameter of approximately20 mm and is formed of polycarbonate. FIG. 6 is a graph showing mirrorflatness in a case where the horizontal axis represents the ratio of thecenter hole diameter with respect to the circumscribed circle diameterbased on the data shown in FIG. 5.

In comparing FIGS.4 and 5, the centrifugal force deformation of the foursided polygon mirror 1001 formed of polycarbonate is approximately 10times compared to that of the four sided polygon mirror 1001 formed ofaluminum. As shown in FIG. 7, this difference is due to Young's modulusand density of each of the four sided polygon mirrors.

Although a flatness of 0.15 μm is normally required for a four sidedpolygon mirror, it is preferred that the ratio of the center holediameter with respect to the circumscribed circle diameter is 10%-40% soas to reduce centrifugal force deformation as shown in FIG. 6.Furthermore, the centrifugal force deformation can be minimized bysetting the ratio around an optimum value of 30%. In a case of using aresin material such as polycarbonate as the base material of the polygonmaterial where Young's modulus of a typical resin material is 1-10 GPa(Giga Pascals), it preferable for the ratio of the center hole diameterto be 10-40%. Furthermore, the centrifugal force deformation can beminimized by setting the ratio of the center hole diameter around anoptimum value of 30%. The reflection surface of the polygon mirror isformed by surface processing in a case where a resin material such aspolycarbonate is used as the base material of the polygon mirror.Accordingly, the base material of the polygon mirror can be easilyfabricated by molding resin material. Thereby, a polygon mirror havinglittle mass can be obtained. As a result, the starting time of theoptical deflector 1000 having the four sided polygon mirror 1001 can beshortened.

It is to be noted that the tendency illustrated in FIG. 6 is not limitedto polycarbonate. In addition to aluminum, a material having Young'smodulus greater than that of aluminum also exhibits the same tendency(although there is some difference in the amount of deformation). Inother words, regardless of material, the ratio of the center holediameter with respect to circumscribed circle diameter of the polygonmirror 1001 is preferred to be in the range 10-40%.

In a case of using a metal material (e.g. aluminum) for forming thepolygon mirror 1001, the polygon mirror 1001 may be formed directly, forexample, by performing a cutting process or a grinding process.Considering the fact that Young's modulus of a typical metal material isin the range 60-220 GPa and that a metal material with high Young'smodulus has a large density, there may be a case where the effect ofhigh Young's modulus is cancelled out. Nevertheless, the metal materialcan attain a centrifugal force deformation that is substantially thesame level as that of high purity aluminum.

As another example of the material of the polygon mirror 1001, a ceramicmaterial having Young's modulus greater than aluminum may be used. Sincethe ceramic material has Young's modulus of 200-400 GPa, the absoluteamount of deformation can be reduced. This increases the degree offreedom in designing the center hole of the polygon mirror 1001. In thecase where ceramic material is used as the base material of the polygonmirror 1001, the reflection surface of the polygon mirror 1001 is formedby surface processing. In such case, since the base material hasextremely high hardness, the reflection surface is resistant toscratches. This makes the optical deflector 1000 suitable for recycling.

[Optical Deflector]

FIG. 8 is a schematic diagram showing an optical deflector 1 used in acolor image forming apparatus according to a related art case (same asFIG. 1 in Japanese Laid-Open Patent No. 2003-177346). The differencebetween the optical deflector 1000 according to an embodiment of thepresent invention and the optical deflector 1 of the related art caseshown in FIG. 8 is mainly the configuration of the polygon mirror of therotary member. Therefore, the configuration of the rotary member 1002according to an embodiment of the present invention is described below.

FIG. 9A is a cross-sectional view showing an optical deflector (polygonscanner) 1000 including a rotary member 1002 having a polygon mirror1001 according to an embodiment of the present invention. FIG. 9B is aperspective view showing the optical deflector (polygon scanner) 1000including the rotary member 1002 having the polygon mirror 1001according to an embodiment of the present invention. The polygon mirror1001 has four polygon mirror reflection surfaces arranged about arotational axis direction of the rotary member 1002 in which eachpolygon mirror reflection surface has effective reflection areas 1001 a,1001 b separated from each other in the rotational axis direction of therotary member 1002. Plural laser beams A, B, C, and D (each laser beamcorresponding to a predetermined color) are incident on respective fourreflection surfaces being axially opposite to each other. The incidentlaser beams A, B, C, and D are deflected by the effective reflectionareas 1 a, 1 b of the four reflection surfaces to thereby scan, forexample, a photoconductor, at high speed.

The rotary member 1002 of the polygon scanner 1000 includes, forexample, a bearing shaft 1010 formed of a martensitic stainless steel, aflange 1030 fixed to the bearing shaft 1010 by shrinkage fit, and thepolygon mirror 1001 fixed to the flange 1030. The polygon mirror 1001has four sides (four reflection surfaces) with respect to the rotationdirection, in which each side includes the effective reflection areas 1a, 1 b. A rotor magnet 1011 is fixed to an inner surface of a lower partof the flange 1030. The rotor magnet 1011, together with a stator core(not shown), form an outer rotor type brushless motor. For example, amartensitic stainless steel (e.g. SUS420J2) is a material suitable forthe bearing shaft 1010 since it can be hardened and have its surfacehardness increased, and also since it has satisfactory wear resistance.

Each reflection surface of the polygon mirror includes the effectivereflection areas 1001 a, 1001 b and a middle area which is not used fordeflecting laser beams incident on the reflection surface. The effectivereflection areas 1001 a, 100 b, and the middle area included in thereflection surface are formed on a single continuous plane. It is to benoted that the distance between the upper and lower laser beam L isdecided according to the vertical space of the fθ lens through which thelaser beams are transmitted after being deflected.

As described above, with the four sided configuration of the polygonmirror 1001, the amount of air resistance during rotation of the polygonmirror 1001 can be reduced considerably. Therefore, unlike the six sidedpolygon mirror, there is no need to cut out an intermediate part betweenits plural reflection surfaces separated in the rotational axisdirection. Furthermore, since the polygon mirror 1001 according to anembodiment of the present invention can be formed in an extremely smallsize, the amount of material used in forming the polygon mirror 1001 canbe reduced. Furthermore, the step of processing the mirror surfaces isrequired to be performed only on four sides rather than on six sides.Furthermore, the area for processing each mirror surface on each side ofthe four sided polygon mirror 1001 is smaller than the area forprocessing each mirror surface on each side of the six sided polygonmirror. As a result, the four sided configuration of the polygon mirror1001 can reduce environmental burdens (e.g. consumption of energy andmaterial) during the stage of manufacturing the optical deflector 1000.Furthermore, the four sided configuration of the polygon mirror 1001 canalso reduce consumption of electric power and noise created by mirrorsagainst the air flow during the stage of operating the optical deflector1000. Furthermore, since the polygon mirror 1001 is formed in a smallsize and the moment of inertia of the rotary member 1002 including thepolygon mirror 1001 is small, the starting time for the polygon mirror1001 to reach a predetermined number of rotations (rotational speed) forperforming the scanning can be shortened. Thereby, the electric powerconsumed when starting the optical deflector 1000 can be reduced.

As shown in FIGS.9A and 9B, the polygon mirror 1001 is fixed to theflange 1003 in a manner in which an inner circumferential portion of theflange 1030 is engaged to each ridge line part (outer ridge line part)between two of the four reflection surfaces of the polygon mirror 1001.In order to reduce the inconsistency of the reflection surfaces, theridge line part of the highly precisely formed polygon mirror 1001 isused as an engagement part. Accordingly, the optical deflector 1000requires no engagement part dedicated for engaging the polygon mirror1001 to the flange 1003. Thus, the small-sized polygon mirror 1001 canbe easily positioned and fixed.

[Optical Scanning Apparatus]

Next, an optical scanning apparatus 2000 according to an embodiment ofthe present invention is described. FIG. 10 is a schematic diagramshowing a main part of an optical scanning apparatus 2000 including thepolygon mirror 1001 of the optical deflector 1000 according to anembodiment of the present invention. The optical scanning apparatus 2000in this embodiment of the present invention is a single beam typescanning apparatus. Although four of these optical scanning apparatuses2000 are used for a tandem type color image forming apparatus, FIG. 10shows a single optical scanning apparatus 2000 for the sake ofconvenience.

The optical scanning apparatus 2000 includes a light source 101, acoupling lens 102, an aperture 103, a cylindrical lens 104, the polygonmirror 1001, lenses 106, 107, a mirror 108, a photoconductor 109, amirror 110, a lens 111, and a light receiving element 112.

The light source 101 includes a semiconductor laser device that emitslight for performing optical scanning. The coupling lens 102 is foradjusting the light emitted from the light source 101 in correspondenceto subsequent optical systems. The aperture 103 is for forming the lightbeam into a predetermined shape for performing optical scanning. Thecylindrical lens 104 is for condensing an incident light beam(s) in asub-scanning direction. The polygon mirror 1001 included in the opticaldeflector 1000 deflects incident beams with its reflection surfaces. Thelenses 106, 107 are for imaging the light beams onto the photoconductor109. The mirror 108 is for bending (deflecting) the optical paths of thelight beams and guiding the light beams to the photoconductor 109. Thephotoconductor 109 forms an electrostatic latent image in accordancewith the light beams irradiated thereto. The mirror 110 and the lens 111condense the light beams onto the light receiving element 112. The lightreceiving element 112 is a photo-detection device such as a photodiode.

The light beam emitted from the light source (semiconductor laserdevice) 101 includes a divergent pencil of rays that are coupled tosubsequent optical systems by the coupling lens 102. The shape of thecoupled beam differs depending on the optical characteristic of thesubsequent optical system. For example, the coupled beam may be, forexample, a pencil of rays having weak divergence, a pencil of rayshaving weak convergence, or a pencil of rays that are parallel. When thebeam transmitted through the coupling lens 102 passes through an openingpart of the aperture 103, the peripheral parts of the pencil of rayswith weak light intensity are blocked so that the coupled beam istransformed into a predetermined shape. Then, the beam is incident onthe cylindrical lens 104 which is a linear imaging optical system. Thecylindrical lens 104, which is shaped substantially as a semicircularcylinder, directs rays without power (not oriented for refraction) in amain scanning direction, condenses incident rays with positive power ina sub-scanning direction, and directs the condensed light to thevicinity of the reflection surfaces of the polygon mirror 1001 of theoptical deflector 1000.

Along with the rotation of the polygon mirror rotated at a constantvelocity, the beam reflected by the reflection surfaces of the polygonmirror 1001 is deflected at a constant angular velocity while beingtransmitted through two lenses 106, 107 that serve as a scanning opticalsystem. The optical path of the transmitted beam is bent by the bendingmirror 108 and condensed as a beam spot onto a target scanning surfaceof the photoconductor 109 so as to scan the target scanning surface.Before the beam scans the photoconductor 109, the beam is incident onthe mirror 110 and condensed to the light receiving element 112 by thelens 111. The timing for writing on the photoconductor 109 is determinedby a control part (not shown) in accordance with the output of the lightreceiving element 112.

Hence, the optical deflector 1000 according to this embodiment of thepresent invention can be used in a single beam type optical scanningapparatus 2000. With the single beam type optical scanning apparatus2000 using the optical deflector 1000 according to an embodiment of thepresent invention, the scanning beam can maintain a constant shape,noise can be reduced, and environmental burdens from the manufacturingstage to the operating stage (e.g. consumption of material and energy)can be reduced owing to the highly precise configuration of thereflection surfaces of the polygon mirror 1001 of the optical deflector1000.

[Multi-Beam Optical Scanning Apparatus]

FIG. 11 is a schematic diagram showing a main part of an opticalscanning apparatus 3000 including the polygon mirror 1001 of the opticaldeflector 1000 according to another embodiment of the present invention.The optical scanning apparatus 3000 in this embodiment of the presentinvention is a multi-beam type scanning apparatus. Although four ofthese optical scanning apparatuses 3000 are used for a tandem type colorimage forming apparatus, FIG. 11 shows a single optical scanningapparatus 3000 for the sake of convenience. In FIG. 11, like componentsare denoted by like reference numerals as of FIG. 10 and are not furtherexplained.

A light source 101A is a semiconductor laser array having four lightsources ch1-ch4 that are evenly spaced in a single row. Although thefour light sources ch1-ch4 according to this embodiment of the presentinvention are arranged in a sub-scanning direction, the semiconductorlaser array 101A may be tilted so that the light sources ch1-ch4 arearranged in a main scanning direction.

The four beams emitted from the four light sources ch1-ch4 include. adivergent pencil of rays having the major axes of their elliptic farfield patterns oriented in the main scanning direction. The four beamsare coupled to subsequent optical systems by the coupling lens (sharedcoupling lens) 102. The shape of the coupled beams differs depending onthe optical characteristic of the subsequent optical systems. Forexample, the coupled beams may be a pencil of rays having weakdivergence, a pencil of rays having weak convergence, or a pencil ofrays that are parallel.

The four beams transmitted through the coupling lens 102 are transformedinto a predetermined shape by the aperture 103. Then, each of the beamsis converged in a sub-scanning direction by the cylindrical lens (sharedlinear imaging optical system) 104. Then, the four beams converged inthe sub-scanning direction are separated from each other in thesub-scanning direction and respectively imaged as a long linear image inthe main scanning direction at the vicinity of the reflection surfacesof the polygon mirror.

The four beams deflected at a constant angular velocity by thereflection surfaces of the polygon mirror 1001 are transmitted throughthe two lenses (scanning optical system) 106, 107 and have their opticalpaths bent by the bending mirror 108. The four beams, having theiroptical paths bent by the bending mirror 108, are condensed as four beamspots separated in the sub-scanning direction onto the target scanningsurface of the photoconductor 109 so as to simultaneously scan thetarget scanning surface as four scanning lines.

Before scanning the photoconductor 109, each of the beams is incident onthe mirror 110 and condensed to the light receiving element 112 by thelens 111. The timing for writing on the photoconductor 109 with the fourbeams is determined by a control part (not shown) in accordance with theoutput of the light receiving element 112.

The scanning optical system according to this embodiment of the presentinvention is for simultaneously condensing the four beams deflected bythe polygon mirror 1001 (optical deflector 1000) in the form of fourbeam spots onto the target scanning surface of the photoconductor 109.The scanning optical system includes two lenses 106, 107.

Hence, the optical deflector 1000 according to this embodiment of thepresent invention can be used in a multi-beam type optical scanningapparatus 3000. With the multi-beam type optical scanning apparatus 3000using the optical deflector 1000 according to this embodiment of thepresent invention, the scanning beams can maintain constant shape, noisecan be reduced, and environmental burdens from the manufacturing stageto the operating stage (e.g. consumption of material and energy) can bereduced owing to the highly precise configuration of the reflectionsurfaces of the polygon mirror 1001 of the optical deflector 1000.

[Image Forming Apparatus]

Next, an image forming apparatus 4000 according to an embodiment of thepresent invention is described. FIG. 12 is a schematic diagram showingan exemplary configuration of a tandem type full color laser printer(image forming apparatus) 4000 including the optical deflector 1000according to an embodiment of the present invention.

A conveyor belt 202 for conveying a transfer sheet (not shown) fed froma sheet feeding cassette 201 is provided in a horizontal position at alower part of the image forming apparatus 4000. A photoconductor 203Yfor yellow (Y), a photoconductor 203M for magenta (M), a photoconductor203C for cyan (C), and a photoconductor for black (K) are arranged atequal intervals on the conveyor belt 202 in this order from an upstreamside of the conveyor belt 202. It is to be noted that the letters Y, M,C, and K are added to reference numerals where colors are to bedistinguished.

The photoconductors 203Y, 203M, 203C, and 203K are formed with the samediameter. Each of the photoconductors 203Y, 203M, 203C, and 203K issurrounded by components arranged in an order corresponding to anelectrophotographic process.

For example, the photoconductor 203Y is surrounding by a charger(electrified body) 204Y, an optical scanning apparatus 205Y, adeveloping apparatus 206Y, a transferring charger 207Y, and a cleaningapparatus 208Y. In other words, the target irradiation surfaces (targetscanning surfaces) of the photoconductors 203Y, 203M, 203C, and 203K areprovided in a one on one relationship with respect to the opticalscanning apparatuses 205Y, 205M, 205C, and 205K.

Furthermore, in the periphery of the conveyor belt 202, resist rollers209 and a belt charger 210 are provided at a position upstream withrespect to the photoconductor 205Y. Moreover, a belt separating charger,static eliminating charger 212, and a cleaning apparatus 213 areprovided at a position downstream with respect to the photoconductor205K.

Furthermore, a fixing apparatus 214 is provided downstream of theconveying direction with respect to the belt separating charger 211. Thefixing apparatus 214 is connected to a sheet discharge tray 215 viasheet discharge rollers 216.

In a case of forming an image(s) in a full color mode (multi-color mode)with the above-described configuration, an electrostatic latent image isformed on each of the photoconductors 203Y, 203M, 203C, and 203K byscanning a light beam from the corresponding one of the optical scanningapparatuses 205Y, 205M, 205C, and 205K based on image signals of thecorresponding one of the colors Y, M, C, and K. These electrostaticlatent images are formed into toner images by being developed withtoners of corresponding color. The toner images are orderly transferredin an overlapping manner (superposed) onto a transfer sheet beingelectrostatically attracted and conveyed on the conveyor belt 202. Thetoner images overlappingly transferred to the transfer sheet are fixedto the transfer sheet by the fixing apparatus 214. Thereby, a full colorimage is formed on the transfer sheet. Then, the transfer sheet isdischarged to the sheet discharge tray 215 by the sheet dischargingrollers 216.

In a case of forming an image(s) in a black mode (single color mode)with the above-described configuration, an electrostatic latent image isformed only on the photoconductor 203K by scanning a light beam from asingle optical scanning apparatus 205K based on image signalscorresponding to black (K) while the other photoconductors 203Y, 203M,203C and corresponding components are in a non-operating state.

The electrostatic latent image is formed into a toner image by beingdeveloped with a black toner. The toner image is transferred to atransfer sheet being electrostatically attracted and conveyed on theconveyor belt 202. The toner image being transferred to the transfersheet is fixed to the transfer sheet by the fixing apparatus 214.Thereby, a monochrome image is formed on the transfer sheet. Then, thetransfer sheet is discharged to the sheet discharge tray 215 by thesheet discharging rollers 216.

Hence, the optical deflector 1000 according to this embodiment of thepresent invention can be used in a tandem type full color laser printer(image forming apparatus). With the tandem type full color laser printerusing the optical deflector 1000 according to an embodiment of thepresent invention, a single optical deflector is shared by opticalscanning apparatuses 205Y, 205M, 205C, and 205K. Accordingly, thescanning beam can maintain a constant shape, noise can be reduced, andenvironmental burdens from the manufacturing stage to the operatingstage (e.g. consumption of material and energy) can be reduced.

Further, the present invention is not limited to these embodiments, butvariations and modifications may be made without departing from thescope of the present invention.

The present application is based on Japanese Priority Application No.2005-372744 filed on Dec. 26, 2005, with the Japanese Patent Office, theentire contents of which are hereby incorporated by reference.

What is claimed is:
 1. An optical deflector including a rotary membersupported by a bearing shaft and rotatively driven by a motor fordeflecting a plurality of laser beams separated from each other in arotational axis direction of the rotary member, the optical deflectorcomprising: a polygon mirror having four sides arranged about therotational axis direction; each of the four sides is a plane ofcontinuous material having a plurality of effective reflection areasseparated from each other in the rotational axis direction by anon-reflective region; the plurality of laser beams incident on thepolygon mirror are parallel, and the plurality of laser beams areperpendicular to a rotational axis of the polygon mirror; each of thefour sides includes a first effective reflection area, a secondeffective reflection area, and a middle area, the middle area includingthe non-reflective region which does not deflect any of the plurality oflaser beams; and a distance, in which the first effective reflectionarea and the second effective reflection area are separated, issubstantially equivalent to a distance between an upper laser beam and alower laser beam of the plurality of laser beams.
 2. The opticaldeflector as claimed in claim 1, wherein the polygon mirror is formed ofa material having a Young's modulus in a range of 60 to 220 GPa.
 3. Theoptical deflector as claimed in claim 2, wherein aluminum is a mainconstituent of the material of the polygon mirror.
 4. The opticaldeflector as claimed in claim 1, wherein the four sides of the polygonmirror are formed by surface processing a material having a Young'smodulus in a range of 200 to 400 GPa.
 5. The optical deflector asclaimed in claim 1, wherein the polygon mirror has a center hole,wherein a ratio of the diameter of the center hole with respect to acircumscribed circle diameter of the polygon mirror is in a range of 10to 40 percent.
 6. The optical deflector as claimed in claim 5, whereinthe four sides of the polygon mirror are formed by surface processing amaterial having a Young's modulus in a range of one to ten GPa.
 7. Theoptical deflector as claimed in claim 1, wherein the polygon mirror isfixed to the rotary member by an engagement portion formed at an outerside of the polygon mirror.
 8. The optical deflector as claimed in claim1, further comprising: a rotating assembly configured to be rotativelydriven by a motor and to rotate the mirror upon the rotational axis, therotating assembly including a bearing shaft, and a flange configured toattach the mirror to the bearing shaft, and the flange is fixed to themirror by an engagement portion formed on an outer side of the polygonmirror, wherein the outer side faces away from the rotational axis, andthe outer side is parallel to the rotational axis, wherein side edges ofthe polygon mirror separate the four sides from each other and the sideedges are parallel to the rotational axis, and wherein the engagementportion formed at the outer side of the polygon mirror includes at leasttwo of the side edges.
 9. An optical scanning apparatus including anoptical system for scanning a scanning line on a target scanning surfaceby guiding a beam of a semiconductor laser to the target scanningsurface and forming a beam spot on the target scanning surface, theoptical scanning apparatus comprising: the optical deflector as claimedin claim 1 for deflecting the beam to the target scanning surface. 10.An image forming apparatus for forming a visible image from a latentimage formed on a photoconductor by scanning a light beam on aphotosensitive surface of the photoconductor, the image formingapparatus comprising: the optical scanning apparatus as claimed in claim9.
 11. An optical scanning apparatus including an optical system forscanning a plurality of scanning lines on a target scanning surface byguiding a plurality of beams of a semiconductor laser to the targetscanning surface and forming a plurality of beam spots on the targetscanning surface, the optical scanning apparatus comprising: the opticaldeflector as claimed in claim 1 for deflecting the plurality of beams tothe target scanning surface.
 12. An image forming apparatus for forminga visible image from a latent image formed on a photoconductor byscanning a light beam on a photosensitive surface of the photoconductor,the image forming apparatus comprising: the optical scanning apparatusas claimed in claim
 11. 13. The optical deflector as claimed in claim 1,wherein side edges of the polygon mirror separate the four sides fromeach other and the side edges are parallel to the rotational axis. 14.The optical deflector as claimed in claim 1, further comprising: arotating assembly configured to be rotatively driven by a motor and torotate the mirror upon the rotational axis, the rotating assemblyincluding a bearing shaft, and a flange configured to attach the mirrorto the bearing shaft, and the flange is fixed to the mirror by anengagement portion formed on an outer side of the polygon mirror,wherein the outer side faces away from the rotational axis, and theouter side is parallel to the rotational axis, and wherein theengagement portion includes at least two of the four sides.
 15. Theoptical deflector as claimed in claim 1, wherein a length of the polygonmirror in the rotational axis direction is shorter than a width of thesides of the polygon mirror.
 16. The optical deflector as claimed inclaim 1, wherein a length of the polygon mirror in the rotational axisdirection is shorter than the diameter of a circumscribed circle of thepolygon mirror.
 17. The optical deflector as claimed in claim 1, whereinthe first and second effective reflection areas are rectangular shapedand have short axes thereof in a direction parallel to the rotationalaxis direction.
 18. The optical deflector as claimed in claim 1, whereineach of the four sides is a single continuous plane of the material. 19.The optical deflector as claimed in claim 1, wherein widths, in therotational axis direction, of the first and second effective reflectionareas are shorter than a width, in the rotational axis direction, of thenon-reflective region of the middle area.